CCIE за год. Цикл 7. Тема 40. Вопросы.

1. Дайте объяснение header compression.

Header compression is a mechanism that compresses the header in a data packet before the packet is transmitted. Header compression reduces network overhead and speeds up the transmission of packets. Header compression also reduces the amount of bandwidth consumed when the packets are transmitted.

2. Какие виды header compression использует cisco.

TCP header compression
RTP header compression

3. Объясните принцип работы header compression в RTP

RTP header compression compresses the RTP header (that is, the combined IP, UDP, and RTP segments) in an RTP packet. RTP header compression identifies the RTP traffic and then compresses the IP header portion of the RTP packet. The IP header portion consists of an IP segment, a UDP segment, and an RTP segment. The minimal 20 bytes of the IP segment, combined with the 8 bytes of the UDP segment, and the 12 bytes of the RTP segment, create a 40-byte IP/UDP/RTP header. The RTP header portion is compressed from 40 bytes to approximately 5 bytes.
RTP header compression is supported on serial interfaces using Frame Relay, HDLC, or PPP encapsulation. It is also supported over ISDN interfaces.

4. Объясните работу Class-Based Header Compression в MQC

Class-based RTP and TCP header compression allows you to configure either RTP or TCP header compression for a specific class within a policy map (sometimes referred to as a traffic policy). You configure the class and the policy map by using the Modular Quality of Service (QoS) Command-Line Interface (CLI) (MQC). The MQC is a CLI that allows you to create classes within policy maps (traffic policies) and then attach the policy maps to interfaces (or subinterfaces). The policy maps are used to configure and apply specific QoS features (such as RTP or TCP header compression) to your network.

5. Назовите преимущества использования Class-Based Header Compression

Class-based header compression allows you to compress (and then decompress) a subset of the packets on your network. Class-based header compression acts as a filter; it allows you to specify at a much finer level the packets that you want to compress. For example, instead of compressing all RTP (or TCP) packets that traverse your network, you can configure RTP header compression to compress only those packets that meet certain criteria (for example, protocol type «ip» in a class called «voice).»

6. Для чего используется Voice Adaptive Traffic Shaping?

Voice Adaptive Traffic Shaping is only acailable with MQC-based Frame-Relay traffic shaping, and only with LLQ enabled in sharper-queue. The goal of VATS is to throttle the PVC’s sending rate once the system detected voice packets in the priority queue. The VATS feature is useful in the oversubscription scenaiio, where the customer sends traffic over the CIR. When the voice packet comes in the priority queue. the system will signal the shaper to slow its rate down to ensure router will never drop or delay any voice packets.

7. Для чего используется Voice Adaptive Fragmentation?

Frame Relay voice-adaptive fragmentation enables a router to fragment large data packets whenever
packets (usually voice) are detected in the low latency queueing priority queue or H.323 call setup
signaling packets are present. When there are no packets in the priority queue for a configured period of
time and signaling packets are not present, fragmentation is stopped. Frame Relay voice-adaptive fragmentation can be used in conjunction with or independent of
voice-adaptive traffic shaping.
To use voice-adaptive fragmentation, you must also have end-to-end fragmentation configured in a map
class or on the interface.

CCIE за год. Цикл 7. Тема 39. Вопросы.

1. Дайте объяснение понятию Feasibility Condition.

The feasibility condition is a sufficient condition for loop freedom in EIGRP-routed network. It is used to select the successors and feasible successors that are guaranteed to be on a loop-free route to a destination.
If, for a destination, a neighbor router advertises a distance that is strictly lower than our feasible distance, then this neighbor lies on a loop-free route to this destination.

2. Дайте объяснение понятию Feasible Distance.

Feasible distance is the best metric along a path to a destination network, including the metric to the neighbor advertising that path.

3. Дайте объяснение понятию Successor.

The best path learned through EIGRP to a destination network is known as the Successor route.

A feasible successor is a path whose reported (advertised) distance is less than the feasible distance (current best path).

4. Дайте объяснение понятию Advertised Distance.

Reported (Advertised) distance is the total metric along a path to a destination network as advertised by an upstream neighbor.

5. Объясните выражение: “The Feasibility Condition cannot guarantee that all loop-free paths will be accepted”

It should be pointed out, however, that the EIGRP Feasibility Condition is not perfect:
— The Feasibility Condition can guarantee that any path with a loop will not be accepted
— The Feasibility Condition cannot guarantee that all loop-free paths will be accepted.

Because of how varied the EIGRP metric can be, there is no perfect formula that would guarantee denying all looped-paths and accepting all loop-free paths. Between the choices of accidentally denying a loop-free path or accidentally accepting a looped-path, the latter is far more dangerous. As such, EIGRP (correctly) took the more conservative approach, and the Feasibility Condition’s main purpose is to guarantee that a looped-path will not be accepted.

6. Дайте описание формуле композитной метрики EIGRP.

EIGRP composite cost metric = 256*((K1*Bw) + (K2*Bw)/(256 – Load) + (K3*Delay)*(K5/(Reliability + K4)))

bandwidth — The minimum bandwidth of the route, in kilobits per second. It can be 0 or any positive integer. The bandwidth for the formula is scaled and inverted by the following formula:(10^7/minimum bandwidth (Bw) in kilobits per second)
delay — Route delay, in tens of microseconds.
reliability — The likelihood of successful packet transmission, expressed as a number between 0 and 255, where 255 means 100 percent reliability and 0 means no reliability.
load — The effective load of the route, expressed as a number from 0 to 255 (255 is 100 percent loading).
mtu — The minimum maximum transmission unit (MTU) size of the route, in bytes. It can be 0 or any positive integer.

7. Дайте объяснение тому, что cisco не рекомендует менять значения EIGRP LOAD и RELIABILITY

Надёжность (reliability) и загрузка линка (load) являются динамическими параметрами, поэтому эти значения пересчитываются только при изменении в сети.

8. Почему не рекомендуется менять метрику Bandwidth?

Для изменения метрики обычно меняют delay, так как bandwidth влияет на QoS, кроме этого, изменение bandwidth не всегда меняет метрику (если наихудший линк не изменился).

9. Дайте объяснение применению Wide Metric.

The EIGRP composite cost metric (calculated using the bandwidth, delay, reliability, load, and K values) is not scaled correctly for high-bandwidth interfaces or Ethernet channels, resulting in incorrect or inconsistent routing behavior. The lowest delay that can be configured for an interface is 10 microseconds. As a result, high-speed interfaces, such as 10 Gigabit Ethernet (GE) interfaces, or high-speed interfaces channeled together (GE ether channel) will appear to EIGRP as a single GE interface. This may cause undesirable equal-cost load balancing. To resolve this issue, the EIGRP Wide Metrics feature supports 64-bit metric calculations and Routing Information Base (RIB) scaling that provide the ability to support interfaces (either directly or via channeling techniques like port channels or ether channels) up to approximately 4.2 terabits.

10. Дайте объяснение equal load balancing.

The routes have to have same metric to be put in routing table as equal cost routes.
Default number of equal cost routes for EIGRP is 4.
Can increase them up to 16 using command below.
conf t
router eigrp 5
maximum-paths 16

11. Дайте объяснение unequal load balancing.

This is where you can have several routes in a routing table which are not of equal cost.
Use command below to specify unequal cost load balancing.

conf t
router eigrp 5
variance 2

The value 2 in variance means that the metric ratio of second route divided by lowest metric (first route) is less than or equal to 2.

The routes have to pass the feasibility condition too.

12. Дайте объяснение RIB_Scale

Because the Wide Metrics composite value can well result in a number wider than 32 bits while the routing table (Routing Information Base, RIB) is capable of handling only 32-bit metrics, the Wide Metrics composite value has to be downscaled before the route can be passed down to the RIB in IOS. This is done by dividing the Wide Metrics composite value by a factor configured in the metric rib-scale EIGRP command. The
default value is 128 and can be configured in the range 1–255. Note that this downscaled value is not used by EIGRP in any way. EIGRP makes all its path selections based on the Wide Metrics composite value; only after a best path toward a destination is selected, its composite metric value is downscaled as the route is installed to the RIB.

CCIE за год. Цикл 7. Тема 38. Вопросы.

1. Для чего используется IP SLA?

IP SLAs allows Cisco customers to analyze IP service levels for IP applications and services, to increase productivity, to lower operational costs, and to reduce the frequency of network outages. IP SLAs uses active traffic monitoring—the generation of traffic in a continuous, reliable, and predictable manner—for measuring network performance. Using IP SLAs, service provider customers can measure and provide service level agreements, and enterprise customers can verify service levels, verify outsourced service level agreements, and understand network performance. IP SLAs can perform network assessments, verify quality of service (QoS), ease the deployment of new services, and assist administrators with network troubleshooting. IP SLAs can be accessed using the Cisco software commands or Simple Network Management Protocol (SNMP) through the Cisco Round-Trip Time Monitor (RTTMON) and syslog Management Information Bases (MIBs).

2. Какой порт при настройке IP SLA не должен быть заблокирован?

IP SLA responder Control port — UDP 1967

3. Назовите параметры интервала и расписания выполнения IP SLA по-умолчанию

frequency — default is 60 seconds
ip sla monitor schedule
life — default is 3600 seconds (1 hour)
ageout — default is 0 seconds (never ages out)

4. Для чего используется Enhanced Object Tracking

This feature provides a more complete alternative to the Hot Standby Routing Protocol (HSRP) tracking mechanism. which allows you to track the line-protocol state of an interface. If the line protocol state of an interface goes down, the HSRP priority of the interface is reduced and another HSRP device with a higher priority becomes active. Theenhanced object tracking feature separates the tracking mechanism from HSRP and creates a separate, standalone tracking process that can be used by processes other than HSRP. This allows tracking other objects in addition to the interface line-protocol state. Aclient process, such as HSRP, can register an interest in tracking objects and request notification when the tracked object changes state.This feature increases the availability and speed of recovery of a routing system and decreases outages and outage duration.

5. Каким образом можно комбинировать несколько треков в Enhanced Object Tracking?

track <number> list boolean <and | or>

and —Specify that the list is up if all objects are up or down if one or more objects are down.
or —Specify that the list is up if one object is up or down if all objects are down.